Thanks Richard. I was looking at those but wondered whether they would be as accurate as the plug in types. However the reviews are generally positive so I might try to hunt one down in the local shops (although a quick Google shows a few out of stock issues).
While I am on that issue I have promised myself that I will support local shops more. I was tempted by a bargain and ordered a set of £4 strings (reduced from £7.50 in a sale), the supplier has sent them by Home Delivery Network. They have left calling cards but will only deliver between 9 and 5 weekdays. I am not taking a day off work for them and the nearest depot if I want to pick them up is a 90 mile 2 hour round trip. I know I could have had them delivered to work but I honestly thought they would just stick them in an envelope and put them in the post. Can't imagine how much those strings have cost to deliver so far.
I'm still waiting for them and in the meantime I walked into a music shop and bought the same strings for just under £7, lesson learnt I think.
Rant over.
Paul